The present invention relates to a semiconductor light-emitting device such as a refractive index waveguide laser or a double-heterojunction light-emitting diode and a method of manufacturing the same and, more particularly, to a semiconductor light-emitting device in which an active region is surrounded by a semiconductor layer having an energy gap wider than that of the active region and a method of manufacturing the same.
Various types of semiconductor light-emitting devices having a double-heterojunction structure have recently been developed. In the semiconductor lightemitting devices of this type, it is important to satisfy the following conditions A to C.
A. A current is efficiently concentrated only in a light-emitting region or an active region which is controlled to have a very small area so as to improve a light-emitting efficiency.
B. An electrode is formed over a wide region so as to decrease the contact resistance.
C. When high-speed modulation is required as in the case of a light-emitting device for optical communication, an area of a portion where a p-n junction is formed is minimized so as to decrease a junction capacitance.
An example of a semiconductor light-emitting device for optical communication, which satisfies the above three conditions to some extent, is a mesa laser utilizing a mass transport technique, which is applied to a GaInAsP/InP semiconductor laser (e.g., Y. Hirayama et al. "Low Temperature and rapid mass transport technique for GaInAsP/InP DFB lasers, Inst. Phys. Conf. Ser. No. 79: Chapt 3 Paper presented at Int. Symp. GaAs and Related Compounds Karuizawa, Japan, 1985, p. 175-186). Such a semiconductor light-emitting device is called an MT laser. A method of manufacturing the MT laser and its characteristics will be described below with reference to the accompanying drawings.
FIGS. 1A to 1D are sectional views schematically showing steps of manufacturing a conventional MT laser. First, as shown in FIG. 1A, 3 .mu.m-thick n-type InP buffer layer 2, 0.1 .mu.m-thick undoped GaInAsP active layer 3 having a composition capable of emitting 1.3 .mu.m-band light, 1.5 .mu.m-thick p-type InP cladding layer 4, and 0.8 .mu.m-thick p+-type GaInAsP cap layer 5 having a composition capable of emitting 1.15 .mu.m-band light for realizing good ohmic contact are successively crystalgrown on the surface of (100) plane of n-type InP substrate 1.
Then, as shown in FIG. 1B, selective etching is performed until layer 3 is exposed to form a mesa portion having a width of 15 .mu.m. At this time, if hydrochloric acid is used to remove layer 4, etching can be automatically stopped at layer 3 because of its selectivity.
Subsequently, as shown in FIG. 1C, both sides of layer 3 are etched by an etchant consisting of sulfuric acid + hydrogen peroxide + water (4:1:1) to form a 1 .mu.m-wide active region. At this time, InP is almost not etched, and only quaternary GaInAsP is etched. Although layer 5 is etched, it is etched to an extent of about 1/3 that of layer 3 because of a difference between their compositions. In order to obtain stable fundamental transverse mode oscillation and a low oscillation threshold current, a width of the active region must be accurately controlled to be about 1 .mu.m.
Thereafter, as shown in FIG. 1D, in consideration of confinement of transverse mode light and a sufficient mechanical strength, a deep constricted portion of layer 3 etched as described above is buried with an InP layer to obtain a so-called buried hetero (BH) structure. In the MT laser, an MT technique is used to grow this buried portion. That is, a phenomenon in which the InP is first grown in the constricted portion if phosphorus is added at a high temperature (670.degree. C). Note that if InCl.sub.3 is used as an catalyst, rapid growth can be achieved at a lower temperature.
SiO.sub.2 film 6 as an insulating film is formed throughout the entire surface of the above element, and a window is formed at a contact portion of the insulating film. An AuZn layer is formed on layer 5 as p-side electrode 7 by a lift-off technique, and electrode 7 is heated and alloyed. Thereafter, Au-Cr is deposited on electrode 7 and film 6 to form electrode 8. In addition, n-side electrode 9 is formed on substrate 1, thereby completing the MT laser.
In this MT laser, a current can be concentrated in an active region or layer 3 by a built-in potential difference between GaInAsP of layer 3 and InP of the buried region. In addition, the junction capacitance is small because the junction is formed only at the mesa portion of a 15 .mu.m width. Thus, the MT laser is advantageous for high-speed response. Furthermore, electrode 7 can be formed to have a width of about 10 .mu.m.
However, the MT laser of this type has a problem of controllability with respect to a width of the active region. That is, when the 15 .mu.m-wide active layer is to be selectively etched from the both sides to form a 1 .mu.m-wide active region, it is difficult to stop etching of an active region at a width of 1 .mu.m with high accuracy, and the entire active layer is sometimes etched, resulting in a poor manufacturing yield. This etching controllability becomes poor as a width of the mesa portion is increased, and a mesa width cannot be formed larger than 15 .mu.m. Where the mesa width is 15 .mu.m, in consideration of a mask alignment margin, a mesa width of an ohmic electrode portion must be set below 10 .mu.m. For this reason, the contact resistance cannot be sufficiently decreased. Furthermore, since an area of the InP junction at the buried portion is defined by the width of the mesa portion, it is difficult to form the area narrower than the mesa width.
Note that although the area of the buried portion can be adjusted by controlling a time of the MT step, controllability thereof is very poor. For this reason, the width of the buried InP junction cannot be optimized, e.g., narrowed to decrease the junction capacitance while confining the transverse mode light. Therefore, it is very difficult to obtain higher performance In addition, a carrier concentration at the buried junction portion must be optimized so that the junction capacitance is decreased and a built-in potential at the junction portion is increased to decrease a current leakage, thereby obtaining a high output. However, in the conventional MT technique, since the carrier concentration is not controlled, a concentration at the junction portion cannot be defined, resulting in a serious design problem.
As described above, according to the conventional MT technique, it is difficult to set the width of the active region with high accuracy, and this difficulty prevents high performance of a buried-type semiconductor light-emitting device. In addition, when an area of the buried portion is decreased, a contact area is decreased to increase the contact resistance. Furthermore, when the contact area is increased, the buried area is increased to increase the junction capacitance, and it is difficult to control the width of the active region.